Stress corrosion cracking of beverage mixed gas cylinders
Health and Safety Executive - Safety alert
HID Chemical Industries Division
07 May 2010
Companies involved in the filling and testing of mixed gas cylinders for the beverage industry
Internal stress corrosion cracking of the cylinder wall in gas cylinders containing a mixture of carbon dioxide and pressurised nitrogen where there has been a potential for liquids to enter the cylinder during use. Stamp markings on the shoulders of cylinders that are excessively deep or sharp.
In March 2010 a man lost a part of his leg when a beverage gas cylinder that he had just finished filling spontaneously exploded into four parts. This was the first fill of the cylinder after it had passed its periodic examination at an approved Inspection Body.
Early investigations suggest strongly that the cylinder failed by stress corrosion cracking starting inside the cylinder and that there were clear indications of internal corrosion of the cylinder walls.
Although the failure started at a large pre-existing crack-like defect, it appears to have continued across the thicker shoulder section of the cylinder through an area with heavy 'XXX' stamp markings. The investigation is considering the possibility that the heavy stamp markings contributed to the fragmentation of the cylinder during the failure.
Between 2000 and 2003 there were eight failures of mixed beverage gas cylinders at cylinder filling premises, in clubs and pubs, and at a training centre and a warehouse. None of the incidents resulted in injury as no-one was present at the moment of failure.
The primary cause of these failures was internal stress corrosion cracking of the cylinder wall, caused by ingress of liquids that had entered the cylinder and were not removed prior to the next pressurisation of the cylinder. These failures are discussed in the British Compressed Gas Association publication 'CP32 The Safe Use of N2, CO2 and CO2/N2 Cylinders in the Beverage Dispense Industry: Revision 1: 2008'
In response to this series of failures, HSE also set up a working group with Industry to look into these and further failures. The minutes of most of these meetings are available on the internet - for example the 5th meeting can be found at Minutes of 5th Meeting of the HSE Gas Cylinder Research Steering Committee.
The safeguards put in place at that time to prevent the filling of contaminated cylinders were effective in reducing the risks of beverage gas cylinders failure by stress corrosion cracking and no incident reports were seen by HSE between February 2003 and March 2010. The current investigation has involved random sampling of beverage gas cylinders and has shown that the safeguards put in place in 2002 have not always been adhered to.
This Safety Alert is issued as an urgent reminder to the beverage gas industry and cylinder inspection bodies of the dangers of allowing mixed gas cylinders to become contaminated internally, and the importance of continuing to apply the necessary measures to properly maintain and inspect beverage gas cylinders in service.
All beverage gas cylinder fillers and Inspection Bodies should reassess the available information and advice in relation to ensuring the safety of cylinders in the beverage gases industry, and in particular to note the following.
Mixed gas cylinder fillers
Those organisations involved in the filling of mixed gas cylinders for the beverage gas industry should ensure that, in addition to the requirements set out in BSEN1919 and BSEN1920, immediately prior to filling any cylinder with carbon dioxide/nitrogen mixtures that the internal surfaces are completely free of any contamination, particularly liquids such as beer, soft drink concentrate and cleaning fluid. TD5/044 gives a hierarchy in the methods of to detection that can be used. These are:
- Check to make sure that residual pressure remains in the cylinder. The use of the RPV aids this process.
- Weight check designed to detect very low levels of contamination. However cylinder tare weights are normally only specified to 100 grams and it has been shown that as little as 5 grams of water could be enough to destroy a gas cylinder. For a weight check to be effective it must address these factors and be sensitive enough to register these levels of contamination.
- Moisture detection; however there are limitations in its use. Moisture detection may be appropriate for on line monitoring of the gas going into the cylinder but it is an unreliable method of detecting moisture already present inside the cylinder from either rainwater or liquid backfeed.
- An alternative method is cylinder evacuation by use of a vacuum pump. This is a very reliable method of detecting free moisture within a cylinder if the correct equipment is available.
Cylinders that cannot be proven to be free of internal contamination should be closely examined internally. This requires the safe removal of the cylinder valve which must be replaced using the correct tools and torque settings. By-products of this procedure include increased wear on the valve threads and increases in the risk of moisture ingress whilst the valve is removed. This method also relies on the person carrying out the inspection being capable of detecting small quantities of moisture, so he/she must have attested eyesight and a good source of lighting the inside of the cylinder.
Where contamination is found then the cylinder should be sent to an approved gas cylinder Inspection Body for thorough examination. It should be noted that if valves are removed and refitted by the filler then the correct torque must be applied when tightening the valve back into the cylinder neck - see BSENISO133341.
Cylinders filled with mixed carbon dioxide and nitrogen and not fitted with an operational residual pressure valve should have a maximum periodicity between thorough examinations of 5 years.
Only cylinders that have no internal contamination may be filled with a mixture of nitrogen and carbon dioxide.
Inspection Bodies should closely examine any cylinder found to contain internal contamination, and initially reject the cylinder if any corrosion can be seen.
Cylinders initially rejected on visual examination can be returned to service if the Inspection Body can demonstrate that there is no stress corrosion cracking within the cylinder wall and that the corrosion seen is within the limits set by the reject criteria in BSEN1968 or BSEN1802.
The Inspection Body should carefully check the form of all stamp markings to ensure that stamping is not excessively deep and has used well radiused lettering. Where stamp markings appear to be of excessive depth or made with sharp tools then there should be a close examination of the stamped area for any signs of cracking. Where the depth of the stamp exceeds the reject criteria given in Appendix C of BSEN1968 for cut or gouge or if a crack is present then the cylinder should be rejected. Where stamp marks are not radiused, see BS EN ISO 13769:2009 paragraph 4.2, then the cylinder should be subjected to further testing or rejected.
Relevant legal documents:
The Carriage of Dangerous Goods and Use of Transportable Pressure Equipment Regulations 2009. SI 2009 No. 1348
- BS EN 1968:2002 Transportable gas cylinders - Periodic inspection and testing of seamless steel gas cylinders.
- BS EN 1802:2002 Transportable gas cylinders - Periodic inspection and testing of seamless aluminium alloy gas cylinders.
- BS EN 1919:2000 Transportable gas cylinders - Cylinders for liquefied gases. Inspection at time of filling.
- BS EN 1920:2000 Transportable gas cylinders - Cylinders for compressed gases. Inspection at time of filling.BS EN ISO 13769:2009 Gas cylinders - Stamp marking
- BS EN ISO13341:1998 - Transportable gas cylinders. Fitting of valves to gas cylinder
- British Compressed Gas Association publication 'CP32 The Safe Use of N2, CO2 and CO2/N2 Cylinders in the Beverage Dispense Industry: Revision 1: 2008'
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